Use the formula to evaluate the series
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This is a geometric series with starting value a=-3, ratio r=-2.

The sum to n terms is a(r^n - 1)/(r-1).

Therefore substituting a=-3 and r=-2, this is -3((-2)^n - 1)/-3=(-2)^n - 1.

For example S₇=-128-1=-129.

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